Diversity techniques have been used for some time in mobile and other radio communications to combat fading fluctuation. Well-known diversity techniques include space diversity and frequency diversity, and known techniques for obtaining a frequency diversity effect include multi-carrier and frequency hopping (FH). Frequency hopping includes fast frequency hopping (FFH), where hopping is carried out one or more times per data symbol, and slow frequency hopping (SFH), where the frequency is hopped for each burst formed on the basis of a signal comprising two or more data symbols. Fast frequency hopping in particular can give an extremely stable transmission path since a frequency diversity effect is obtained for each symbol.
FIG. 34 is a block diagram showing an example of a conventional frequency diversity transmitter and receiver using fast frequency hopping.
This conventional system has both a transmitter and a receiver. The transmitter is provided with a quadrature modulator 201, a frequency synthesizer 202, a frequency controller 203 and a bandpass filter 204. The quadrature modulator 201 modulates the input symbol sequence with carrier frequencies generated by the frequency synthesizer 202. The frequencies generated by the frequency synthesizer 202 are controlled by the frequency controller 203, with K different frequencies being generated in a prescribed order during the interval of one symbol of the input symbol sequence, where K is an integer equal to or greater than 2.
The output of the quadrature modulator 201 passes through the bandpass filter 204 and is transmitted from an antenna. The modulated signal thus transmitted is a signal wherein each symbol comprises K chips. It will be assumed here that K=4.
The receiver is then provided with K=4 systems each comprising a mixer 205, a local oscillator 206, a bandpass filter 207 and a square-law detector 208. It is also provided with a combiner 209 which combines the signals from the four systems. The signal received by the antenna is distributed to the four mixers 205. Local frequencies from the local oscillators 206 are respectively supplied to these four mixers 205. The outputs of the mixers 205 are input to bandpass filters 207, and the chip signals are extracted from the fast frequency-hopped signal. Square-law detection of these extracted chip signals is carried out by square-law detectors 208 for level recovery. The combiner 209 outputs the level sum of the chips over each symbol.
Demodulation systems in fast frequency hopping may have either coherent detection or non-coherent detection. Because non-coherent detection does not look at the carrier phase of the chips, the transmission characteristics are inferior to those obtained with coherent detection, where the phase of the carrier is included in the detection process. In the case of binary phase-shift keying (BPSK), for example, it is known that to obtain the same error rate, the carrier-to-noise ratio (CNR) of non-coherent detection will be 6 dB worse than that of coherent detection. However, in a practical mobile radio communication, carrier synchronization is difficult to achieve due to the rapidity of the fading fluctuation in the transmission path, and so the aforementioned non-coherent detection method has been conventionally employed.
In the multi-carrier technique, which is another method for obtaining a frequency diversity effect, one and the same symbol is modulated with different carrier frequencies. Diversity transmission can be achieved if these are converted to the baseband at the receiver side and optimally combined, since the carriers will not all be at a low level simultaneously in a frequency selective fading channel. With this method, however, since the same signal is transmitted using a plurality of carriers, the modulation bandwidth shows an equivalent increase and so the frequency spectrum is not efficiently used.